1. Field of the Invention
The present invention relates to a velocimeter and a position information detection apparatus and, more particularly, to a velocimeter and a position information detection apparatus which can be suitably applied to a Doppler velocimeter for accurately measuring the moving velocity and the moving distance of a moving object or fluid in a non-contact manner.
2. Related Background Art
As is well known, a Doppler velocimeter is an apparatus for measuring the moving velocity and the moving distance of a moving object or fluid by utilizing the fact that a light beam is modulated by diffraction or interference generated upon radiation of a laser beam onto a moving object or fluid.
FIG. 1A shows an example of diffraction which occurs when a laser beam I from a laser diode 1 is incident on a transmission type diffraction grating 10 in an alignment direction t of grating lines with a grating line pitch dg. If m represents the order (1, 2, . . .) of diffraction, and .lambda. represents the wavelength of light, a diffraction angle .theta..sub.0 is given by: EQU sin .theta..sub.0 =m.lambda./dg (1)
The diffraction angle of .+-.n-th order light components (n=1, 2, . . .) other than 0-th order light is given by: EQU sin .theta..sub.0 =.+-.n .lambda./dg (2)
FIG. 1B is a view showing a state wherein .+-.n-th order diffracted light components 5a and 5b transmitted through the diffraction grating 10 are reflected by mirrors 6a and 6b to have an incident angle .theta..sub.0 on an object 7 to be measured as a moving object. A Doppler frequency F detected by a photodetector 9 which receives divergent light from the object 7 to be measured, which is moving at a velocity V is given by: EQU F=2V sin .theta..sub.0 /.lambda.=2n V/dg (3)
Thus, the frequency F does not depend on the wavelength .lambda. of the laser beam I, is inversely proportional to the grating line pitch dg of the diffraction grating 10, and is proportional to the velocity V of the object 7 to be measured. Since the grating line pitch dg is sufficiently stable, the Doppler frequency F is a frequency which is proportional to only the velocity V of the object 7 to be measured.
In FIG. 1B, a lens 8 focuses light reflected by the object 7 to be measured on the photodetector 9, and a velocimeter body 61 holds the above-mentioned constituting members. The mirrors 6a and 6b are separated by a distance L, and the diffraction grating 10 and the object 7 to be measured are separated by a measurement distance h. The diffraction grating 10 may be replaced by a reflection type diffraction grating.
A conventional laser Doppler velocimeter shown in FIG. 1C includes an optical system, which radiates a laser beam onto the object 7 to be measured, so that an incident angle .theta. changes in correspondence with a change in wavelength .lambda. of the laser beam, and sin .theta./.lambda. becomes constant. The optical system is constituted by the diffraction grating 10 for forming .+-.n-th order diffracted light components (n=1, 2, 3, . . .) by diffracting the laser beam, and two lenses 11L and 12L having an equal focal length f. The distance between the two lenses 11L and 12L is set to be twice the focal length f. These constituting members are held on a velocimeter body 101 while maintaining a predetermined positional relationship.
With this arrangement, a shift between two crossing light beams upon variation of the wavelength can be minimized. A laser beam from the laser diode 1 is collimated by a collimator lens 2 to a collimated beam 3 having a beam spot size of 1.2 mm, and the collimated beam 3 is incident in the alignment direction of the transmission type diffraction grating 10 having a grating line pitch of 3.2 .mu.m.
When .+-.1st-order diffracted light components 5a and 5b obtained from the transmission type diffraction grating 10 are incident on the convex lens 11L having the focal length f, light beams 13a and 13b are obtained, as shown in FIG. 1C. When these light beams 13a and 13b are incident on the other convex lens 12L separated from the lens 11L by 2f, collimated beams 14a and 14b are obtained again. Then, the collimated beams 14a and 14b having a beam spot size of 1.2 mm are radiated onto the object 7 to be measured, which is moving in the direction of an arrow 7a in FIG. 1C, at an angle equal to the diffraction angle .theta. from the above-mentioned diffraction grating 10.
Since divergent light from the object 7 to be measured is efficiently focused on a light-receiving portion 9a of the photodetector 9 via the convex lens 12L and the focusing lens 8, the Doppler frequency F can be detected by the following equation: EQU F=2V/dg (4)
Assuming that the wavelength .lambda. of the laser diode 1 changes, .theta. varies in correspondence with dg sin .theta.=.lambda., but the Doppler signal does not change as in the above-mentioned apparatus. In this apparatus, the spot positions of two light beams can be set to be immovable. More specifically, since the object 7 to be measured is immovable, no positional shift between the spots occurs, and a proper beam crossing state is always maintained. Since a distance a between the diffraction grating 10 and the convex lens 11L&lt;a distance b between the convex lens 12L and the object 7 to be measured, the distance b is relatively large, and a large working distance can be assured, resulting in a high degree of freedom in terms of velocity design. In addition, temperature dependence of measurement accuracy is very small, and high-accuracy velocity measurement can be realized.
However, in the conventional laser Doppler velocimeter, an object to be measured must be present within a range where interference fringes are formed upon crossing of laser beams. In the arrangement of the conventional laser Doppler velocimeter shown in FIG. 1C, the beam spot size of the laser beam is set to be 1.2 mm, and the laser crossing angle is set to be 12.degree.. For this reason, a range where interference fringes to be sufficiently measured are formed, i.e., the measurable range is 20.+-.1 mm from the convex lens 12L by actual measurement. For example, when the moving velocity of a steel plate is to be measured, if the steel plate suffers a three-dimensional surface pattern or a warp, a depth nonuniformity of only 2 mm is allowed.